Introduction

Members of the family Herpesviridae replicate their genomes in the infected cell nucleus and have a characteristic virion morphology, which consists of the envelope, tegument, capsid and core (Davison and Clements, 1997). An extensive description of virion structure is given in Chapter 3. The present chapter focuses on the viral genome, which occupies the core of the virus particle. Electron microscopy of negatively stained capsids gives the impression that the core consists of the viral DNA molecule wrapped toroidally around a protein spindle (Furlong et al., 1972). Images reconstructed from electron micrographs of virions frozen in ice in the absence of stain, a technique by which morphology is better preserved, show that the core consists of the DNA packed at high density in liquid crystalline form, probably as a spool lacking a spindle (Booy et al., 1991; Zhou et al., 1999).

Herpesvirus genomes consist of linear, double-stranded DNA molecules that range in size from about 125 to 240kbp and in nucleotide composition from 32 to 75% G+C, depending on the virus species (Honess, 1984). The genome termini are not covalently closed (as in the Poxviridae; Moss, 2001) or covalently linked to a protein (as in the Adenoviridae; Shenk, 2001). In those herpesvirus genomes that have been examined in sufficient detail, unpaired nucleotides are present at the termini; for example, HSV-1, VZV and HCMV have a single 3′-overhanging nucleotide at each terminus (Mocarski and Roizman, 1982; Davison, 1984; Tamashiro and Spector, 1986).